Heterophasic propylene copolymers are well established in a lot of applications, such as molding applications, films, wires and cables or pipes, because of their good stiffness/impact ratio together with good flowability. For example heterophasic polymers are used for the production of car exteriors and interiors, in particular dashboards, door claddings, consoles, trims and the like as well as in the packaging area for food and medicals.
Heterophasic polymers are polymers having a matrix phase and a dispersed phase. The matrix phase is usually a propylene homopolymer or copolymer and the dispersed phase is often an elastomeric material, like a propylene copolymer comprising ethylene and/or other higher α-olefin(s).
Many different types of heterophasic systems have been described depending on the individual desire.
For instance in WO 01/19915 A1 an impact resistant polypropylene is disclosed. This product is featured by reasonable impact and stiffness behavior. However the material is difficult to obtain due to the very low melt flow rates be it measured on the end product or during the different process steps.
WO 03/106553 A1 is also directed to an impact resistant polypropylene. This impact resistant polypropylene has been produced in the presence of a metallocene catalyst, leading to a narrow molecular weight distribution. Further the good impact properties of the product are owed to the fact that nucleating agents have been added.
However the trend in industries is nowadays towards polymer materials with good flowability as a fast throughput of the material in the process lines is desired.
It is well known in the art a higher melt flow rate (MFR) improves the processability. Unfortunately known impact modified polypropylenes with rather high melt flow rate (MFR), respectively low molecular weight, tend to be intrinsically very brittle material. In order to improve its toughness high amounts of elastomeric polymer must be added. This in turn has an adverse effect on flow, stiffness and hexane extractables (hexane-solubles).
Moreover a high melt flow rate (MFR) means normally also a higher content of the low molecular weight (LMW) fraction in the polymer. The low molecular weight tail of the polymer comprises oligomers. Accordingly also the oligomer content in polymer compositions increases with increasing MFR. The oligomers thus belong to a generally known group of compounds, namely volatiles. Such volatiles contribute also to the so called “fogging” problem, i.e. these compounds tend to separate, e.g. evaporate, from the polymer material and cause fogging on the surrounding surfaces e.g. on car windows.
For these reasons, although much development work has been done in the field of heterophasic polypropylene copolymers there is a continuous need for alternative or improved heterophasic polypropylene copolymers, which fulfill the various demanding customer requirements in many end application areas of polymer materials, such as packaging, including food and medical packaging, fiber, pipe and automobile industry.